Advanced link tracking for virtual cluster switching

ABSTRACT

One embodiment of the present invention provides a switch system. The switch includes a port that couples to a server hosting a number of virtual machines. The switch also includes a link tracking module. During operation, the link tracking module determines that reachability to at least one end host coupled to a virtual cluster switch of which the switch is a member is disrupted. The link tracking module then determines that at least one virtual machine coupled to the port is affected by the disrupted reachability, and communicates to the server hosting the affected virtual machine about the disrupted reachability.

RELATED APPLICATIONS

This application is a continuation application of application Ser. No.13/092,460, entitled “Advanced Link Tracking for Virtual ClusterSwitching,” by inventors Suresh Vobbilisetty and Phanidhar Koganti,filed 22 Apr. 2011, which claims the benefit of U.S. ProvisionalApplication No. 61/352,255, entitled “Advanced Link Tracking For VirtualCluster Switching,” by inventors Suresh Vobbilisetty, and PhanidharKoganti, filed 7 Jun. 2010, the disclosure of which is incorporated byreference herein.

The present disclosure is related to U.S. patent application Ser. No.12/725,249, entitled “REDUNDANT HOST CONNECTION IN A ROUTED NETWORK,” byinventors Somesh Gupta, Anoop Ghanwani, Phanidhar Koganti, and ShunjiaYu, filed 16 Mar. 2010; and

U.S. patent application Ser. No. 13/087,239, entitled “VIRTUAL CLUSTERSWITCHING,” by inventors Suresh Vobbilisetty and Dilip Chatwani, filed14 Apr. 2011;

the disclosures of which are incorporated by reference herein.

BACKGROUND Field

The present disclosure relates to network design. More specifically, thepresent disclosure relates to a method and system for advanced linkstate monitoring in a virtual cluster switch.

Related Art

The relentless growth of the Internet has brought with it an insatiabledemand for bandwidth. As a result, equipment vendors race to buildlarger, faster, and more versatile switches to move traffic. However,the size of a switch cannot grow infinitely. It is limited by physicalspace, power consumption, and design complexity, to name a few factors.More importantly, because an overly large system often does not provideeconomy of scale due to its complexity, simply increasing the size andthroughput of a switch may prove economically unviable due to theincreased per-port cost.

One way to increase the throughput of a switch system is to use switchstacking. In switch stacking, multiple smaller-scale, identical switchesare interconnected in a special pattern to form a larger logical switch.However, switch stacking requires careful configuration of the ports andinter-switch links. The amount of required manual configuration becomesprohibitively complex and tedious when the stack reaches a certain size,which precludes switch stacking from being a practical option inbuilding a large-scale switching system. Furthermore, a system based onstacked switches often has topology limitations which restrict thescalability of the system due to fabric bandwidth considerations.

In addition, the evolution of virtual computing has placed additionalrequirements on the network. For example, as the locations of virtualservers become more mobile and dynamic, or as the link state within anetwork changes, it is often desirable that the network configurationand the virtual machines can respond to the changes in a timely fashion.However, at present, there are no readily applicable solution that canachieve this goal without using proprietary communication protocols.

SUMMARY

One embodiment of the present invention provides a switch system. Theswitch includes a port that couples to a server hosting a number ofvirtual machines. The switch also includes a link tracking module.During operation, the link tracking module determines that reachabilityto at least one end host coupled to a virtual cluster switch of whichthe switch is a member is disrupted. The link tracking module thendetermines that at least one virtual machine coupled to the port isaffected by the disrupted reachability, and communicates to the serverhosting the affected virtual machine about the disrupted reachability.

In a variation on this embodiment, the link tracking module monitors atleast a link coupled to the switch, a port on the switch, or both.

In a variation on this embodiment, while communicating to the server,the link tracking module communicates with a hypervisor residing in theserver.

In a variation on this embodiment, the switch includes a routingmechanism configured to update a network topology and correspondingreachability information upon detecting a link or port failure.

In a variation on this embodiment, the virtual cluster switch includes anumber of member switches, which are allowed to be coupled in anarbitrary topology. Furthermore, the virtual cluster switch appears tobe a single logical switch.

One embodiment of the present invention provides a system thatfacilitates advanced link tracking. The system includes a first switchcoupled to a first network interface of a server which hosts a number ofvirtual machines and a second switch coupled to a second networkinterface of the server. The first switch comprises a link monitoringmechanism. During operation, the link monitoring mechanism determinesthat reachability via the first switch to at least one end host coupledto the system is disrupted. The link monitoring mechanism furtherdetermines that at least one virtual machine is affected by thedisrupted reachability, and communicates to the server about thedisrupted reachability.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A illustrates an exemplary virtual cluster switch (VCS) system, inaccordance with an embodiment of the present invention.

FIG. 1B illustrates an exemplary VCS system where the member switchesare configured in a CLOS network, in accordance with an embodiment ofthe present invention.

FIG. 2 illustrates the protocol stack within a virtual cluster switch,in accordance with an embodiment of the present invention.

FIG. 3 illustrates an exemplary configuration of a virtual clusterswitch, in accordance with an embodiment of the present invention.

FIG. 4 illustrates an exemplary configuration of how a virtual clusterswitch can be connected to different edge networks, in accordance withan embodiment of the present invention.

FIG. 5A illustrates how a logical Fibre Channel switch fabric is formedin a virtual cluster switch in conjunction with the example in FIG. 4,in accordance with an embodiment of the present invention.

FIG. 5B illustrates an example of how a logical FC switch can be createdwithin a physical Ethernet switch, in accordance with one embodiment ofthe present invention.

FIG. 6 illustrates an exemplary VCS configuration database, inaccordance with an embodiment of the present invention.

FIG. 7 illustrates an exemplary process of a switch joining a virtualcluster switch, in accordance with an embodiment of the presentinvention.

FIG. 8 presents a flowchart illustrating the process of looking up aningress frame's destination MAC address and forwarding the frame in aVCS, in accordance with one embodiment of the present invention.

FIG. 9 illustrates how data frames and control frames are transportedthrough a VCS, in accordance with one embodiment of the presentinvention.

FIG. 10 illustrates a logical VCS access layer (VAL) which facilitatesadvanced link tracking, in accordance with one embodiment of the presentinvention.

FIG. 11 illustrates an exemplary configuration of advanced link trackingin a VCS, in accordance with one embodiment of the present invention.

FIG. 12 illustrates an example where advanced link tracking allowsvirtual machines to re-route egress traffic when a link fails, inaccordance with one embodiment of the present invention.

FIG. 13 presents a flowchart illustrating the process of advance linktracking, in accordance with one embodiment of the present invention.

FIG. 14 illustrates an exemplary switch that facilitates virtual clusterswitching and advanced link tracking, in accordance with one embodimentof the present invention.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the invention, and is provided in the context ofa particular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andscope of the present invention. Thus, the present invention is notlimited to the embodiments shown, but is to be accorded the widest scopeconsistent with the claims.

Overview

In embodiments of the present invention, the problem of dynamicallynotifying end hosts about the link state within a network is solved bynotifying the physical end host and the corresponding virtual machinesabout the disrupted reachability via advanced link tracking. Alarge-scale logical switch (referred to as a “virtual cluster switch” orVCS herein) is formed using a number of smaller physical switches. Theautomatic configuration capability provided by the control plane runningon each physical switch allows any number of switches to be connected inan arbitrary topology without requiring tedious manual configuration ofthe ports and links. This feature makes it possible to use many smaller,inexpensive switches to construct a large cluster switch, which can beviewed as a single logical switch externally. The VCS facilitate fastdetection of any internal link failure or external port failure. Whensuch a failure affects the reachability of one or more hosts via a givenport, the affected hosts (and the associated virtual machines) arenotified via their corresponding ingress ports. This feature allows theaffected virtual machines to be re-configured to use a different ingressport on the VCS, thereby bypassing the failure. In this disclosure, thedescription in conjunction with FIGS. 1-9 is associated with the generalarchitecture of VCS, and the description in conjunction with FIG. 10 andonward provide more details on the advanced link tracking mechanism.

It should be noted that a virtual cluster switch is not the same asconventional switch stacking. In switch stacking, multiple switches areinterconnected at a common location (often within the same rack), basedon a particular topology, and manually configured in a particular way.These stacked switches typically share a common address, e.g., IPaddress, so they can be addressed as a single switch externally.Furthermore, switch stacking requires a significant amount of manualconfiguration of the ports and inter-switch links. The need for manualconfiguration prohibits switch stacking from being a viable option inbuilding a large-scale switching system. The topology restrictionimposed by switch stacking also limits the number of switches that canbe stacked. This is because it is very difficult, if not impossible, todesign a stack topology that allows the overall switch bandwidth toscale adequately with the number of switch units.

In contrast, a VCS can include an arbitrary number of switches withindividual addresses, can be based on an arbitrary topology, and doesnot require extensive manual configuration. The switches can reside inthe same location, or be distributed over different locations. Thesefeatures overcome the inherent limitations of switch stacking and makeit possible to build a large “switch farm” which can be treated as asingle, logical switch. Due to the automatic configuration capabilitiesof the VCS, an individual physical switch can dynamically join or leavethe VCS without disrupting services to the rest of the network.

Furthermore, the automatic and dynamic configurability of VCS allows anetwork operator to build its switching system in a distributed and“pay-as-you-grow” fashion without sacrificing scalability. The VCS'sability to respond to changing network conditions makes it an idealsolution in a virtual computing environment, where network loads oftenchange with time.

Although this disclosure is presented using examples based on theTransparent Interconnection of Lots of Links (TRILL) as the transportprotocol and the Fibre Channel (FC) fabric protocol as the control-planeprotocol, embodiments of the present invention are not limited to TRILLnetworks, or networks defined in a particular Open SystemInterconnection Reference Model (OSI reference model) layer. Forexample, a VCS can also be implemented with switches runningmulti-protocol label switching (MPLS) protocols for the transport. Inaddition, the terms “RBridge” and “switch” are used interchangeably inthis disclosure. The use of the term “RBridge” does not limitembodiments of the present invention to TRILL networks only. The TRILLprotocol is described in IETF draft “RBridges: Base ProtocolSpecification,” available athttp://tools.ietf.org/html/draft-ietf-trill-rbridge-protocol, which isincorporated by reference herein

The terms “virtual cluster switch,” “virtual cluster switching,” and“VCS” refer to a group of interconnected physical switches operating asa single logical switch. The control plane for these physical switchesprovides the ability to automatically configure a given physical switch,so that when it joins the VCS, little or no manual configuration isrequired.

The term “RBridge” refers to routing bridges, which are bridgesimplementing the TRILL protocol as described in IETF draft “RBridges:Base Protocol Specification.” Embodiments of the present invention arenot limited to the application among RBridges. Other types of switches,routers, and forwarders can also be used.

The terms “frame” or “packet” refer to a group of bits that can betransported together across a network. “Frame” should not be interpretedas limiting embodiments of the present invention to layer-2 networks.“Packet” should not be interpreted as limiting embodiments of thepresent invention to layer-3 networks. “Frame” or “packet” can bereplaced by other terminologies referring to a group of bits, such as“cell” or “datagram.”

VCS Architecture

FIG. 1A illustrates an exemplary virtual cluster switch system, inaccordance with an embodiment of the present invention. In this example,a VCS 100 includes physical switches 101, 102, 103, 104, 105, 106, and107. A given physical switch runs an Ethernet-based transport protocolon its ports (e.g., TRILL on its inter-switch ports, and Ethernettransport on its external ports), while its control plane runs an FCswitch fabric protocol stack. The TRILL protocol facilitates transportof Ethernet frames within and across VCS 100 in a routed fashion (sinceTRILL provides routing functions to Ethernet frames). The FC switchfabric protocol stack facilitates the automatic configuration ofindividual physical switches, in a way similar to how a conventional FCswitch fabric is formed and automatically configured. In one embodiment,VCS 100 can appear externally as an ultra-high-capacity Ethernet switch.More details on FC network architecture, protocols, naming/addressconventions, and various standards are available in the documentationavailable from the NCITS/ANSI T11 committee (www.t11.org) and publiclyavailable literature, such as “Designing Storage Area Networks,” by TomClark, 2nd Ed., Addison Wesley, 2003, the disclosures of which areincorporated by reference in their entirety herein.

A physical switch may dedicate a number of ports for external use (i.e.,to be coupled to end hosts or other switches external to the VCS) andother ports for inter-switch connection. Viewed externally, VCS 100appears to be one switch to a device from the outside, and any port fromany of the physical switches is considered one port on the VCS. Forexample, port groups 110 and 112 are both VCS external ports and can betreated equally as if they were ports on a common physical switch,although switches 105 and 107 may reside in two different locations.

The physical switches can reside at a common location, such as a datacenter or central office, or be distributed in different locations.Hence, it is possible to construct a large-scale centralized switchingsystem using many smaller, inexpensive switches housed in one or morechassis at the same location. It is also possible to have the physicalswitches placed at different locations, thus creating a logical switchthat can be accessed from multiple locations. The topology used tointerconnect the physical switches can also be versatile. VCS 100 isbased on a mesh topology. In further embodiments, a VCS can be based ona ring, fat tree, or other types of topologies.

In one embodiment, the protocol architecture of a VCS is based onelements from the standard IEEE 802.1Q Ethernet bridge, which isemulated over a transport based on the Fibre Channel Framing andSignaling-2 (FC-FS-2) standard. The resulting switch is capable oftransparently switching frames from an ingress Ethernet port from one ofthe edge switches to an egress Ethernet port on a different edge switchthrough the VCS.

Because of its automatic configuration capability, a VCS can bedynamically expanded as the network demand increases. In addition, onecan build a large-scale switch using many smaller physical switcheswithout the burden of manual configuration. For example, it is possibleto build a high-throughput fully non-blocking switch using a number ofsmaller switches. This ability to use small switches to build a largenon-blocking switch significantly reduces the cost associated switchcomplexity. FIG. 1B presents an exemplary VCS with its member switchesconnected in a CLOS network, in accordance with one embodiment of thepresent invention. In this example, a VCS 120 forms a fully non-blocking8×8 switch, using eight 4×4 switches and four 2×2 switches connected ina three-stage CLOS network. A large-scale switch with a higher portcount can be built in a similar way.

FIG. 2 illustrates the protocol stack within a virtual cluster switch,in accordance with an embodiment of the present invention. In thisexample, two physical switches 202 and 204 are illustrated within a VCS200. Switch 202 includes an ingress Ethernet port 206 and aninter-switch port 208. Switch 204 includes an egress Ethernet port 212and an inter-switch port 210. Ingress Ethernet port 206 receivesEthernet frames from an external device. The Ethernet header isprocessed by a medium access control (MAC) layer protocol. On top of theMAC layer is a MAC client layer, which hands off the informationextracted from the frame's Ethernet header to a forwarding database(FDB) 214. Typically, in a conventional IEEE 802.1Q Ethernet switch, FDB214 is maintained locally in a switch, which would perform a lookupbased on the destination MAC address and the VLAN indicated in theEthernet frame. The lookup result would provide the corresponding outputport. However, since VCS 200 is not one single physical switch, FDB 214would return the egress switch's identifier (i.e., switch 204'sidentifier). In one embodiment, FDB 214 is a data structure replicatedand distributed among all the physical switches. That is, every physicalswitch maintains its own copy of FDB 214. When a given physical switchlearns the source MAC address and VLAN of an Ethernet frame (similar towhat a conventional IEEE 802.1Q Ethernet switch does) as being reachablevia the ingress port, the learned MAC and VLAN information, togetherwith the ingress

Ethernet port and switch information, is propagated to all the physicalswitches so every physical switch's copy of FDB 214 can remainsynchronized. This prevents forwarding based on stale or incorrectinformation when there are changes to the connectivity of end stationsor edge networks to the VCS.

The forwarding of the Ethernet frame between ingress switch 202 andegress switch 204 is performed via inter-switch ports 208 and 210. Theframe transported between the two inter-switch ports is encapsulated inan outer MAC header and a TRILL header, in accordance with the TRILLstandard. The protocol stack associated with a given inter-switch portincludes the following (from bottom up): MAC layer, TRILL layer, FC-FS-2layer, FC E-Port layer, and FC link services (FC-LS) layer. The FC-LSlayer is responsible for maintaining the connectivity information of aphysical switch's neighbor, and populating an FC routing informationbase (RIB) 222. This operation is similar to what is done in an FCswitch fabric. The FC-LS protocol is also responsible for handlingjoining and departure of a physical switch in VCS 200. The operation ofthe FC-LS layer is specified in the FC-LS standard, which is availableat http://www.t11.org/ftp/t11/member/fc/ls/06-393v5.pdf, the disclosureof which is incorporated herein in its entirety.

During operation, when FDB 214 returns the egress switch 204corresponding to the destination MAC address of the ingress Ethernetframe, the destination egress switch's identifier is passed to a pathselector 218. Path selector 218 performs a fabric shortest-path first(FSPF)-based route lookup in conjunction with RIB 222, and identifiesthe next-hop switch within VCS 200. In other words, the routing isperformed by the FC portion of the protocol stack, similar to what isdone in an FC switch fabric.

Also included in each physical switch are an address manager 216 and afabric controller 220. Address manager 216 is responsible forconfiguring the address of a physical switch when the switch first joinsthe VCS. For example, when switch 202 first joins VCS 200, addressmanager 216 can negotiate a new FC switch domain ID, which issubsequently used to identify the switch within VCS 200. Fabriccontroller 220 is responsible for managing and configuring the logicalFC switch fabric formed on the control plane of VCS 200.

One way to understand the protocol architecture of VCS is to view theVCS as an FC switch fabric with an Ethernet/TRILL transport. Eachphysical switch, from an external point of view, appears to be a TRILLRBridge. However, the switch's control plane implements the FC switchfabric software. In other words, embodiments of the present inventionfacilitate the construction of an

“Ethernet switch fabric” running on FC control software. This uniquecombination provides the VCS with automatic configuration capability andallows it to provide the ubiquitous Ethernet services in a very scalablefashion.

FIG. 3 illustrates an exemplary configuration of a virtual clusterswitch, in accordance with an embodiment of the present invention. Inthis example, a VCS 300 includes four physical switches 302, 304, 306,and 308. VCS 300 constitutes an access layer which is coupled to twoaggregation switches 310 and 312. Note that the physical switches withinVCS 300 are connected in a ring topology. Aggregation switch 310 or 312can connect to any of the physical switches within VCS 300. For example,aggregation switch 310 is coupled to physical switches 302 and 308.These two links are viewed as a trunked link to VCS 300, since thecorresponding ports on switches 302 and 308 are considered to be fromthe same logical switch, VCS 300. Note that, without VCS, such topologywould not have been possible, because the FDB needs to remainsynchronized, which is facilitated by the VCS.

FIG. 4 illustrates an exemplary configuration of how a virtual clusterswitch can be connected to different edge networks, in accordance withan embodiment of the present invention. In this example, a VCS 400includes a number of TRILL RBridges 402, 404, 406, 408, and 410, whichare controlled by the FC switch-fabric control plane. Also included inVCS 400 are RBridges 412, 414, and 416. Each RBridge has a number ofedge ports which can be connected to external edge networks.

For example, RBridge 412 is coupled with hosts 420 and 422 via 10GEports. RBridge 414 is coupled to a host 426 via a 10GE port. TheseRBridges have TRILL-based inter-switch ports for connection with otherTRILL RBridges in VCS 400. Similarly, RBridge 416 is coupled to host 428and an external Ethernet switch 430, which is coupled to an externalnetwork that includes a host 424. In addition, network equipment canalso be coupled directly to any of the physical switches in VCS 400. Asillustrated here, TRILL RBridge 408 is coupled to a data storage 417,and TRILL RBridge 410 is coupled to a data storage 418.

Although the physical switches within VCS 400 are labeled as “TRILLRBridges,” they are different from the conventional TRILL RBridge in thesense that they are controlled by the FC switch fabric control plane. Inother words, the assignment of switch addresses, link discovery andmaintenance, topology convergence, routing, and forwarding can behandled by the corresponding FC protocols. Particularly, each TRILLRBridge's switch ID or nickname is mapped from the corresponding FCswitch domain ID, which can be automatically assigned when a switchjoins VCS 400 (which is logically similar to an FC switch fabric).

Note that TRILL is only used as a transport between the switches withinVCS 400. This is because TRILL can readily accommodate native Ethernetframes. Also, the TRILL standards provide a ready-to-use forwardingmechanism that can be used in any routed network with arbitrary topology(although the actual routing in VCS is done by the FC switch fabricprotocols). Embodiments of the present invention should be not limitedto using only TRILL as the transport. Other protocols (such asmulti-protocol label switching (MPLS) or Internet Protocol (IP)), eitherpublic or proprietary, can also be used for the transport.

VCS Formation

In one embodiment, a VCS is created by instantiating a logical FC switchin the control plane of each switch. After the logical FC switch iscreated, a virtual generic port (denoted as G_Port) is created for eachEthernet port on the RBridge. A G_Port assumes the normal G_Portbehavior from the FC switch perspective. However, in this case, sincethe physical links are based on Ethernet, the specific transition from aG_Port to either an FC F_Port or E_Port is determined by the underlyinglink and physical layer protocols. For example, if the physical Ethernetport is connected to an external device which lacks VCS capabilities,the corresponding G_Port will be turned into an F_Port. On the otherhand, if the physical Ethernet port is connected to a switch with VCScapabilities and it is confirmed that the switch on the other side ispart of a VCS, then the G_Port will be turned into an E_port.

FIG. 5A illustrates how a logical Fibre Channel switch fabric is formedin a virtual cluster switch in conjunction with the example in FIG. 4,in accordance with an embodiment of the present invention. RBridge 412contains a virtual, logical FC switch 502. Corresponding to the physicalEthernet ports coupled to hosts 420 and 422, logical FC switch 502 hastwo logical F_Ports, which are logically coupled to hosts 420 and 422.In addition, two logical N_Ports, 506 and 504, are created for hosts 420and 422, respectively. On the VCS side, logical FC switch 502 has threelogical E_Ports, which are to be coupled with other logical FC switchesin the logical FC switch fabric in the VCS.

Similarly, RBridge 416 contains a virtual, logical FC switch 512.Corresponding to the physical Ethernet ports coupled to host 428 andexternal switch 430, logical FC switch 512 has a logical F_Port coupledto host 428, and a logical FL_Port coupled to switch 430. In addition, alogical N_Port 510 is created for host 428, and a logical NL_Port 508 iscreated for switch 430. Note that the logical FL_Port is created becausethat port is coupled to a switch (switch 430), instead of a regularhost, and therefore logical FC switch 512 assumes an arbitrated looptopology leading to switch 430. Logical NL_Port 508 is created based onthe same reasoning to represent a corresponding NL_Port on switch 430.On the VCS side, logical FC switch 512 has two logical E_Ports, which tobe coupled with other logical FC switches in the logical FC switchfabric in the VCS.

FIG. 5B illustrates an example of how a logical FC switch can be createdwithin a physical Ethernet switch, in accordance with one embodiment ofthe present invention. The term “fabric port” refers to a port used tocouple multiple switches in a VCS. The clustering protocols control theforwarding between fabric ports. The term “edge port” refers to a portthat is not currently coupled to another switch unit in the VCS.Standard IEEE 802.1Q and layer-3 protocols control forwarding on edgeports.

In the example illustrated in FIG. 5B, a logical FC switch 521 iscreated within a physical switch (RBridge) 520. Logical FC switch 521participates in the FC switch fabric protocol via logical inter-switchlinks (ISLs) to other switch units and has an FC switch domain IDassigned to it just as a physical FC switch does. In other words, thedomain allocation, principal switch selection, and conflict resolutionwork just as they would on a physical FC ISL.

The physical edge ports 522 and 524 are mapped to logical F_Ports 532and 534, respectively. In addition, physical fabric ports 526 and 528are mapped to logical E_Ports 536 and 538, respectively. Initially, whenlogical FC switch 521 is created (for example, during the boot-upsequence), logical FC switch 521 only has four G_Ports which correspondto the four physical ports. These G_Ports are subsequently mapped toF_Ports or E_Ports, depending on the devices coupled to the physicalports.

Neighbor discovery is the first step in VCS formation between twoVCS-capable switches. It is assumed that the verification of VCScapability can be carried out by a handshake process between twoneighbor switches when the link is first brought up.

In general, a VCS presents itself as one unified switch composed ofmultiple member switches. Hence, the creation and configuration of VCSis of critical importance. The VCS configuration is based on adistributed database, which is replicated and distributed over allswitches.

In one embodiment, a VCS configuration database includes a globalconfiguration table (GT) of the VCS and a list of switch descriptiontables (STs), each of which describes a VCS member switch. In itssimplest form, a member switch can have a VCS configuration databasethat includes a global table and one switch description table, e.g.,[<GT><ST>]. A VCS with multiple switches will have a configurationdatabase that has a single global table and multiple switch descriptiontables, e.g., [<GT><ST0><ST1> . . . <STn−1>]. The number n correspondsto the number of member switches in the VCS. In one embodiment, the GTcan include at least the following information: the VCS ID, number ofnodes in the VCS, a list of VLANs supported by the VCS, a list of allthe switches (e.g., list of FC switch domain IDs for all activeswitches) in the VCS, and the FC switch domain ID of the principalswitch (as in a logical FC switch fabric). A switch description tablecan include at least the following information: the IN_VCS flag,indication whether the switch is a principal switch in the logical FCswitch fabric, the FC switch domain ID for the switch, the FC world-widename (WWN) for the corresponding logical FC switch; the mapped ID of theswitch, and optionally the IP address of the switch.

In addition, each switch's global configuration database is associatedwith a transaction ID. The transaction ID specifies the latesttransaction (e.g., update or change) incurred to the globalconfiguration database. The transaction IDs of the global configurationdatabases in two switches can be compared to determine which databasehas the most current information (i.e., the database with the morecurrent transaction ID is more up-to-date). In one embodiment, thetransaction ID is the switch's serial number plus a sequentialtransaction number. This configuration can unambiguously resolve whichswitch has the latest configuration.

As illustrated in FIG. 6, a VCS member switch typically maintains twoconfiguration tables that describe its instance: a VCS configurationdatabase 600, and a default switch configuration table 604. VCSconfiguration database 600 describes the VCS configuration when theswitch is part of a VCS. Default switch configuration table 604describes the switch's default configuration. VCS configuration database600 includes a GT 602, which includes a VCS identifier (denoted asVCS_ID) and a VLAN list within the VCS. Also included in VCSconfiguration database 600 are a number of STs, such as ST0, ST1, andSTn. Each ST includes the corresponding member switch's MAC address andFC switch domain ID, as well as the switch's interface details. Notethat each switch also has a VCS-mapped ID which is a switch index withinthe VCS.

In one embodiment, each switch also has a VCS-mapped ID (denoted as“mappedID”), which is a switch index within the VCS. This mapped ID isunique and persistent within the VCS. That is, when a switch joins theVCS for the first time, the VCS assigns a mapped ID to the switch. Thismapped ID persists with the switch, even if the switch leaves the VCS.When the switch joins the VCS again at a later time, the same mapped IDis used by the VCS to retrieve previous configuration information forthe switch. This feature can reduce the amount of configuration overheadin VCS. Also, the persistent mapped ID allows the VCS to “recognize” apreviously configured member switch when it re-joins the VCS, since adynamically assigned FC fabric domain ID would change each time themember switch joins and is configured by the VCS.

Default switch configuration table 604 has an entry for the mappedIDthat points to the corresponding ST in VCS configuration database 600.Note that only VCS configuration database 600 is replicated anddistributed to all switches in the VCS. Default switch configurationtable 604 is local to a particular member switch.

The “IN_VCS” value in default switch configuration table 604 indicateswhether the member switch is part of a VCS. A switch is considered to be“in a VCS” when it is assigned one of the FC switch domains by the FCswitch fabric with two or more switch domains. If a switch is part of anFC switch fabric that has only one switch domain, i.e., its own switchdomain, then the switch is considered to be “not in a VCS.”

When a switch is first connected to a VCS, the logical FC switch fabricformation process allocates a new switch domain ID to the joiningswitch. In one embodiment, only the switches directly connected to thenew switch participate in the VCS join operation.

Note that in the case where the global configuration database of ajoining switch is current and in sync with the global configurationdatabase of the VCS based on a comparison of the transaction IDs of thetwo databases (e.g., when a member switch is temporarily disconnectedfrom the VCS and re-connected shortly afterward), a trivial merge isperformed. That is, the joining switch can be connected to the VCS, andno change or update to the global VCS configuration database isrequired.

FIG. 7 illustrates an exemplary process of a switch joining a virtualcluster switch, in accordance with an embodiment of the presentinvention. In this example, it is assumed that a switch 702 is within anexisting VCS, and a switch 704 is joining the VCS. During operation,both switches 702 and 704 trigger an FC State Change Notification (SCN)process. Subsequently, both switches 702 and 704 perform a PRE-INVITEoperation. The pre-invite operation involves the following process.

When a switch joins the VCS via a link, both neighbors on each end ofthe link present to the other switch a VCS four-tuple of <Prior VCS_ID,SWITCH_MAC, mappedID, IN_VCS> from a prior incarnation, if any.Otherwise, the switch presents to the counterpart a default tuple. Ifthe VCS_ID value was not set from a prior join operation, a VCS_ID valueof −1 is used. In addition, if a switch's IN_VCS flag is set to 0, itsends out its interface configuration to the neighboring switch. In theexample in FIG. 7, both switches 702 and 704 send the above informationto the other switch.

After the above PRE-INVITE operation, a driver switch for the joinprocess is selected. By default, if a switch's IN_VCS value is 1 and theother switch's IN_VCS value is 0, the switch with IN_VCS=1 is selectedas the driver switch. If both switches have their IN_VCS values as 1,then nothing happens, i.e., the PRE-INVITE operation would not lead toan INVITE operation. If both switches have their IN_VCS values as 0,then one of the switches is elected to be the driving switch (forexample, the switch with a lower FC switch domain ID value). The drivingswitch's IN_VCS value is then set to 1 and drives the join process.

After switch 702 is selected as the driver switch, switch 702 thenattempts to reserve a slot in the VCS configuration databasecorresponding to the mappedID value in switch 704's PRE-INVITEinformation. Next, switch 702 searches the VCS configuration databasefor switch 704's MAC address in any mappedID slot. If such a slot isfound, switch 702 copies all information from the identified slot intothe reserved slot. Otherwise, switch 702 copies the information receivedduring the PRE-INVITE from switch 704 into the VCS configurationdatabase. The updated VCS configuration database is then propagated toall the switches in the VCS as a prepare operation in the database (notethat the update is not committed to the database yet).

Subsequently, the prepare operation may or may not result inconfiguration conflicts, which may be flagged as warnings or fatalerrors. Such conflicts can include inconsistencies between the joiningswitch's local configuration or policy setting and the VCSconfiguration. For example, a conflict arises when the joining switch ismanually configured to allow packets with a particular VLAN value topass through, whereas the VCS does not allow this VLAN value to enterthe switch fabric from this particular RBridge (for example, when thisVLAN value is reserved for other purposes). In one embodiment, theprepare operation is handled locally and/or remotely in concert withother VCS member switches. If there is an un-resolvable conflict, switch702 sends out a PRE-INVITE-FAILED message to switch 704. Otherwise,switch 702 generates an INVITE message with the VCS's merged view of theswitch (i.e., the updated VCS configuration database).

Upon receiving the INVITE message, switch 704 either accepts or rejectsthe INVITE. The INVITE can be rejected if the configuration in theINVITE is in conflict with what switch 704 can accept. If the INVITE isacceptable, switch 704 sends back an INVITE-ACCEPT message in response.The INVITE-ACCEPT message then triggers a final database committhroughout all member switches in the VCS. In other words, the updatedVCS configuration database is updated, replicated, and distributed toall the switches in the VCS.

Layer-2 Services in VCS

In one embodiment, each VCS switch unit performs source MAC addresslearning, similar to what an Ethernet bridge does. Each {MAC address,VLAN} tuple learned on a physical port on a VCS switch unit isregistered into the local Fibre Channel Name Server (FC-NS) via alogical Nx_Port interface corresponding to that physical port. Thisregistration binds the address learned to the specific interfaceidentified by the Nx_Port. Each FC-NS instance on each VCS switch unitcoordinates and distributes all locally learned {MAC address, VLAN}tuples with every other FC-NS instance in the fabric. This featureallows the dissemination of locally learned {MAC addresses, VLAN}information to every switch in the VCS. In one embodiment, the learnedMAC addresses are aged locally by individual switches.

FIG. 8 presents a flowchart illustrating the process of looking up aningress frame's destination MAC address and forwarding the frame in aVCS, in accordance with one embodiment of the present invention. Duringoperation, a VCS switch receives an Ethernet frame at one of itsEthernet ports (operation 802). The switch then extracts the frame'sdestination MAC address and queries the local FC Name Server (operation804). Next, the switch determines whether the FC-NS returns an N_Port oran NL_Port identifier that corresponds to an egress Ethernet port(operation 806).

If the FC-NS returns a valid result, the switch forwards the frame tothe identified N_Port or NL_Port (operation 808). Otherwise, the switchfloods the frame on the TRILL multicast tree as well as on all theN_Ports and NL_Ports that participate in that VLAN (operation 810). Thisflood/broadcast operation is similar to the broadcast process in aconventional TRILL RBridge, wherein all the physical switches in the VCSwill receive and process this frame, and learn the source addresscorresponding to the ingress RBridge. In addition, each receiving switchfloods the frame to its local ports that participate in the frame's VLAN(operation 812). Note that the above operations are based on thepresumption that there is a one-to-one mapping between a switch's TRILLidentifier (or nickname) and its FC switch domain ID. There is also aone-to-one mapping between a physical Ethernet port on a switch and thecorresponding logical FC port.

End-to-End Frame Delivery

FIG. 9 illustrates how data frames and control frames are transported ina VCS, in accordance with an embodiment of the present invention.

In this example, a VCS 930 includes member switches 934, 936, 938, 944,946, and 948. An end host 932 is communicating with an end host 940.Switch 934 is the ingress VCS member switch corresponding to host 932,and switch 938 is the egress VCS member switch corresponding to host938. During operation, host 932 sends an Ethernet frame 933 to host 940.Ethernet frame 933 is first encountered by ingress switch 934. Uponreceiving frame 933, switch 934 first extracts frame 933's destinationMAC address. Switch 934 then performs a MAC address lookup using theEthernet name service, which provides the egress switch identifier(i.e., the RBridge identifier of egress switch 938). Based on the egressswitch identifier, the logical FC switch in switch 934 performs arouting table lookup to determine the next-hop switch, which is switch936, and the corresponding output port for forwarding frame 933. Theegress switch identifier is then used to generate a TRILL header (whichspecifies the destination switch's RBridge identifier), and the next-hopswitch information is used to generate an outer Ethernet header.Subsequently, switch 934 encapsulates frame 933 with the proper TRILLheader and outer Ethernet header, and sends the encapsulated frame 935to switch 936. Based on the destination RBridge identifier in the TRILLheader of frame 935, switch 936 performs a routing table lookup anddetermines the next hop. Based on the next-hop information, switch 936updates frame 935's outer Ethernet header and forwards frame 935 toegress switch 938.

Upon receiving frame 935, switch 938 determines that it is thedestination RBridge based on frame 935's TRILL header. Correspondingly,switch 938 strips frame 935 of its outer Ethernet header and TRILLheader, and inspects the destination MAC address of its inner Ethernetheader. Switch 938 then performs a MAC address lookup and determines thecorrect output port leading to host 940. Subsequently, the originalEthernet frame 933 is transmitted to host 940.

As described above, the logical FC switches within the physical VCSmember switches may send control frames to one another (for example, toupdate the VCS global configuration database or to notify other switchesof the learned MAC addresses). In one embodiment, such control framescan be FC control frames encapsulated in a TRILL header and an outerEthernet header. For example, if the logical FC switch in switch 944 isin communication with the logical FC switch in switch 938, switch 944can sends a TRILL-encapsulated FC control frame 942 to switch 946.Switch 946 can forward frame 942 just like a regular data frame, sinceswitch 946 is not concerned with the payload in frame 942.

Advanced Link Tracking

Today's server virtualization infrastructure (e.g. a Hypervisor, alsocalled virtual machine monitor) typically provides one or more virtualswitches (also called virtual Ethernet bridges, VEBs) within a physicalserver. Each virtual switch serves a number of virtual machines. When anumber of such servers connect to a VCS, the number of communicationsessions among the virtual machines can be quite large. In such anetwork environment, when a network link or port fails, the failurewould typically disrupt the reachability to one or more virtualmachines. This disruption can affect the communication sessions of someof the virtual machines. In conventional networks, such reachabilitydisruption only triggers a topology change and/or MAC address learningupdate in the network, and the source virtual machines are not notifiedabout these updates. Correspondingly, with conventional technologies,there is no way for a Hypervisor to re-configure the connectivity of thevirtual machines absent of some signaling from the network viaproprietary protocols.

Embodiments of the present invention facilitate advanced link trackingby monitoring any reachability disruption in the network and notifyingthe affected hypervisor. In response, the hypervisor can re-configurethe connectivity of the virtual machines under its control to bypass thefailed link or port. In one embodiment, this advanced link trackingfunction can be carried out in a logical VCS access layer.

FIG. 10 illustrates a logical VCS access layer (VAL) which facilitatesadvanced link tracking, in accordance with one embodiment of the presentinvention. In this example, a VCS 1000 is coupled with a number ofphysical server systems, such as system 1002. Each physical serversystem runs a number of virtual machines (VMs, also called virtualservers). For example, system 1002 includes four VMs, one of which is VM1004. A VM may be dedicated to a certain application (e.g., instantmessaging services, directory services, data base applications, etc.)and may have its own requirement on the network. A cluster of VMsrunning certain applications may communicate with another cluster of VMsacross VCS 1000.

The switches within VCS 100 which are coupled externally to the physicalend-host systems form a logical VCS access layer (VAL) 1010. In someembodiments, the advanced link tracking functions can be partly carriedout in VAL 1010. During operation, any link state change that affectsend-host reachability can be notified to the affected end hosts via VAL1010. As described in detail below, when the reachability via an egressport is lost, this information is communicated to the physical serverswhich contain VMs in communication with the affected end hosts.

FIG. 11 illustrates an exemplary configuration of advanced link trackingin a VCS, in accordance with one embodiment of the present invention. Inthis example, a VCS 1100 includes four switches (which can be RBridges),1120, 1122, 1124, and 1126. A physical server 1118 is coupled to bothswitches 1122 and 1124 via two network interface cards (NICs), 1103 and1105, respectively. Physical server 1118 hosts four VMs, 1122, 1124,1126, and 1128, which are managed by a hypervisor 1101. Hypervisor 1101provides two virtual switches, 1102 and 1104. Each VM has two virtualports (VPs), and is coupled to both virtual switches 1102 and 1104 viathe VPs. In other words, each VM within physical server 1118 isdual-homed with virtual switches 1102 and 1104. This configurationprovides redundancy to each VM, so that when one of the physical NICs(i.e., NIC 1103 or 1105) fails, hypervisor 1101 can instruct the VMs touse the other working NIC. During normal operation, for load-balancingpurposes, VMs 1122 and 1124 are configured to communicate via virtualswitch 1102, and VMs 1126 and 1128 are configured to communicate viavirtual switch 1104.

Also coupled to VCS 1100 is physical servers 1117, which has a similarconfiguration as server 1118. Server 1117 includes four VMs, 1132, 1134,1136, and 1138. These four VMs are each dual-homed with virtual switches1142 and 1144, which are provided by hypervisor 1141. Virtual switch1142 is coupled to VCS member switch 1120 via a NIC 1143, and virtualswitch 1144 is coupled to VCS member switch 1126 via a NIC 1145. Duringnormal operation, VMs 1132 and 1134 communicate with VCS 1100 viavirtual switch 1142 and NIC 1143, and VMs 1136 and 1138 communicate withVCS 1100 via virtual switch 1144 and NIC 1145.

Assume that VMs 1122 and 1124 are in communication with VMs 1136 and1138. Since VMs 1136 and 1138 are configured by hypervisor 1141 to usevirtual switch 1144 and NIC 1145, the traffic between VMs 1122 and 1124and VMs 1136 and 1138 is normally carried by VCS member switch 1126.Now, assume the link between switches 1120 and 1126 fails. As a result,VMs 1136 and 1138 can no longer be reached via NIC 1145. In embodimentsof the present invention, this rechability update information is notonly reflected in the VCS topology update (which is handled by therouting protocol within VCS 1100), but also communicated to hypervisor1101 via NIC 1103. This update can allow hypervisor 1101 to quicklyre-configure VMs 1122 and 1124, so that these two VMs use virtual switch1104 and NIC 1105 to access VCS 1100. This way, the traffic from VMs1122 and 1124 can still reach VMs 1136 and 1138 via switch 1124, switch1120, NIC 1143, and virtual switch 1142. The new data path bypasses thefailed link between switches 1120 and 1126. This re-configuration cantake place shortly after the link failure is detected, therebyfacilitating fast recovery at the source VMs.

FIG. 12 illustrates an example where advanced link tracking allowsvirtual machines to re-route egress traffic when a link fails, inaccordance with one embodiment of the present invention. In thisexample, two servers 1202 and 1204 are coupled to a VCS 1200. Server1202 hosts four VMs, 1206, 1208, 1210, and 1212, all of which aredual-homed with virtual switches 1214 and 1216. During operation, VMs1206 and 1208 access VCS 1200 via VS 1214, and VMs 1210 and 1212 accessVCS 1200 via VS 1216. Server 1204 have a similar configuration as server1202. Assume that throughout VCS 1200 there is only one path leadingfrom VS 1214 to VS 1218 in server 1204. Assume further that duringoperation the egress port coupling to VS 1218 in server 1204 fails. As aresult, VS 1218 is no longer reachable from VS 1214. The advanced linktracking mechanism can notify VS 1214 of the lost reachability to VS1218. In one embodiment, VCS 1200 can communicate with a third entitywhich maintains the connectivity-pattern information among all the VMs(such as the vCenter by VMware) to obtain information on the affectedVMs. In further embodiments, VCS 1200 can notify every external port ofthe lost reachability, and let the individual hypervisor to determinewhether re-configuration of the VM-to-VS connectivity is necessary.

FIG. 13 presents a flowchart illustrating the process of advance linktracking, in accordance with one embodiment of the present invention.During operation, the system first detects a link (or port) failure inthe VCS (operation 1302). The system then determines whether the failureaffects reachability of an end host (operation 1304). If the failuredoes not affect reachability of any end host, it is assumed that VCS canrecover from the failure after its topology converges and the routingprotocol updates every switch's forwarding table. If the reachability ofan end host is affected, the system then optionally identifies ingressport(s) which are in communication with the affected end host(s)(operation 1306). Subsequently, the system notifies the end hosts viathe ingress ports of the reachability disruption (operation 1308).

Exemplary VCS Member Switch with Advanced Link Tracking

FIG. 14 illustrates an exemplary VCS member switch, in accordance withone embodiment of the present invention. In this example, the VCS memberswitch is a TRILL RBridge 1400 running special VCS software. RBridge1400 includes a number of Ethernet communication ports 1401, which canbe coupled to one or more servers hosting virtual machines and which cantransmit and receive Ethernet frames and/or TRILL encapsulated frames.Also included in RBridge 1400 is a packet processor 1402, a virtual FCswitch management module 1404, a logical FC switch 1405, a VCSconfiguration database 1406, an advanced link tracking module 1407, anda TRILL header generation module 1408.

During operation, packet processor 1402 extracts the source anddestination MAC addresses of incoming frames, and attaches properEthernet or TRILL headers to outgoing frames. Virtual FC switchmanagement module 1404 maintains the state of logical FC switch 1405,which is used to join other VCS switches using the FC switch fabricprotocols. VCS configuration database 1406 maintains the configurationstate of every switch within the VCS. TRILL header generation module1408 is responsible for generating property TRILL headers for framesthat are to be transmitted to other VCS member switches.

Upon learning about disrupted reachability in the VCS, advanced linktracking module 1407 identifies the port(s) which are affected by thedisruption, and notifies the hypervisor of the disruption. Thisnotification can allow the hypervisor to expedite the re-configurationof the affected VMs and minimize service disruption. Furthermore,advanced link tracking module 1407 also monitors the health of all thelinks corresponding to ports 1401. Upon detection of any link or portfailure, advanced link tracking module 1407 can notify other switches inthe VCS of the link state change and any reachability disruption.

The methods and processes described herein can be embodied as codeand/or data, which can be stored in a computer-readable non-transitorystorage medium. When a computer system reads and executes the codeand/or data stored on the computer-readable non-transitory storagemedium, the computer system performs the methods and processes embodiedas data structures and code and stored within the medium.

The methods and processes described herein can be executed by and/orincluded in hardware modules or apparatus. These modules or apparatusmay include, but are not limited to, an application-specific integratedcircuit (ASIC) chip, a field-programmable gate array (FPGA), a dedicatedor shared processor that executes a particular software module or apiece of code at a particular time, and/or other programmable-logicdevices now known or later developed. When the hardware modules orapparatus are activated, they perform the methods and processes includedwithin them.

The foregoing descriptions of embodiments of the present invention havebeen presented only for purposes of illustration and description. Theyare not intended to be exhaustive or to limit this disclosure.Accordingly, many modifications and variations will be apparent topractitioners skilled in the art. The scope of the present invention isdefined by the appended claims.

What is claimed is:
 1. A switch, comprising: one or more ports; a linktracking module configured to: detect a failure which affectsreachability between the switch and a second switch in a network ofinterconnected switches, wherein the network of interconnected switchesis identified by a fabric identifier; in response to determining thatthe failure affects reachability to an end host, identify a port viawhich the end host is reachable; and use the identified port as anegress port for a notification message comprising information regardingthe failure.
 2. The switch of claim 1, in response to determining thatthe failure does not affect reachability to the end host, the linktracking module is further configured to determine that a routing updatein the network of interconnected switches can resolve the failure. 3.The switch of claim 1, wherein reachability to the end host affectsreachability to a virtual machine hosted in the end host, and whereinthe notification message is destined to a hypervisor running the virtualmachine to allow the hypervisor to reconfigure the virtual machine. 4.The switch of claim 3, wherein the link tracking module is furtherconfigured to construct a second notification message comprisingreachability information associated with the failure, wherein the secondnotification message is destined to a second hypervisor residing in asecond end host.
 5. The switch of claim 3, wherein the reachability tothe virtual machine includes reachability between the virtual machineand a second virtual machine.
 6. The switch of claim 1, wherein the linktracking module is further configured to monitor a link between theswitch and the second switch for failure.
 7. The switch of claim 1,further comprising an encapsulation module configured to encapsulate apacket with an encapsulation header, wherein the encapsulated packet isforwardable in the network of interconnected switches based on theencapsulation header.
 8. A method, comprising: detecting a failure whichaffects reachability between a switch and a second switch in a networkof interconnected switches, wherein the network of interconnectedswitches is identified by a fabric identifier; in response todetermining that the failure affects reachability to an end host,identifying a port via which the end host is reachable; and using theidentified port as an egress port for a notification message comprisinginformation regarding the failure.
 9. The method of claim 8, furthercomprising, in response to determining that the failure does not affectreachability to the end host, determining that a routing update in thenetwork of interconnected switches can resolve the failure.
 10. Themethod of claim 8, wherein reachability to the end host affectsreachability to a virtual machine hosted in the end host, and whereinthe notification message is destined to a hypervisor running the virtualmachine to allow the hypervisor to reconfigure the virtual machine. 11.The method of claim 10, further comprising constructing a secondnotification message comprising reachability information associated withthe failure, wherein the second notification message is destined to asecond hypervisor residing in a second end host.
 12. The method of claim10, wherein the reachability to the virtual machine includesreachability between the virtual machine and a second virtual machine.13. The method of claim 8, further comprising monitoring a link betweenthe switch and the second switch for failure.
 14. The method of claim 8,further comprising encapsulating a packet with an encapsulation header,wherein the encapsulated packet is forwardable in the network ofinterconnected switches based on the encapsulation header.
 15. Anon-transitory computer-readable storage medium storing instructionsthat when executed by a computer cause the computer to perform a method,the method comprising: detecting a failure which affects reachabilitybetween a switch and a second switch in a network of interconnectedswitches, wherein the network of interconnected switches is identifiedby a fabric identifier; in response to determining that the failureaffects reachability to an end host, identifying a port via which theend host is reachable; and using the identified port as an egress portfor a notification message comprising information regarding the failure,wherein the notification message is destined to a hypervisor running avirtual machine to allow the hypervisor to reconfigure the virtualmachine.
 16. The non-transitory computer-readable storage medium ofclaim 15, wherein the method further comprises, in response todetermining that the failure does not affect reachability to the endhost, determining that a routing update in the network of interconnectedswitches can resolve the failure.
 17. The non-transitorycomputer-readable storage medium of claim 15, wherein reachability tothe end host affects reachability to a virtual machine hosted in the endhost, and wherein the notification message is destined to a hypervisorrunning the virtual machine to allow the hypervisor to reconfigure thevirtual machine.
 18. The non-transitory computer-readable storage mediumof claim 17, wherein the method further comprises constructing a secondnotification message comprising reachability information associated withthe failure, wherein the second notification message is destined to asecond hypervisor residing in a second end host.
 19. The non-transitorycomputer-readable storage medium of claim 15, wherein the method furthercomprises monitoring a link between the switch and the second switch forfailure.
 20. The non-transitory computer-readable storage medium ofclaim 15, wherein the method further comprises encapsulating a packetwith an encapsulation header, wherein the encapsulated packet isforwardable in the network of interconnected switches based on theencapsulation header.